/Engine/Plugins/Editor/UVEditor/Source/UVEditorTools/Public/Selection/UVToolSelectionAPI.h
A tool that returns false here will have the unset element selections (ie selections of triangles that don't have associated UV's) cleared before its invocation. Returning true will keep these unset element selections, but the tool will be responsible for dealing with them and avoiding any invalid states.
virtual bool SupportsUnsetElementAppliedMeshSelections() const